1. Field of the Disclosure
The disclosure relates generally to downhole formation analysis and provides an apparatus and methods for demagnetizing a wellbore.
2. Summary of the Related Art
It is well known that drilling mud may contain magnetic particles which may influence formation surveys taken by monitoring while drilling (MWD) directional sensors. This effect is described in IADC/SPE 87169 and SPE 71400. The magnetic particles are mainly due to wear on the casing and on the drill-string. The particles are known to collect especially in synthetic oil-based mud that is recycled and used for a long period of time and even at different locations. Magnetic filters (i.e., ditch magnets) have been used to filter magnetic particles but generally can not filter magnetic particles out of the mud that are too small to be attracted to the magnetic filter. Mud contaminated with magnetic steel particles can invade the formation and can also collect in a filter cake. This collection of magnetic particles typically creates a zone close to the wellbore wall containing magnetic material which can not circulate with the rest of the drilling mud. When permanent magnets from nuclear magnetic resonance (NMR) tools, magnetic fishing tools or casing collar locaters are moved through the wellbore, these collected particles can become magnetized. Because of magnetic remanance, the particles maintain their magnetization and provide a magnetic field for a long time. The magnetic field created by these magnetically aligned particles can influence the measurements obtained using magnetic tools deployed from a wireline or drill string, such as magnetic azimuth measurements or other magnetic measurements of the earth formation. A similar problem may occur in a cased wellbore in which the casing is made of a magnetic material such as steel. The residual magnetization of the casing may affect the performance of sensors such as magnetometers that are conveyed through the casing at a later time. Thus, there is a need to remove or reduce the effects on magnetic measurements of magnetic field remanance due to surrounding materials in a wellbore.